Sheets are utilized in various fields in today's society; and sheets of various materials and compositions are produced industrially. Various standardized goods of sheets are provided in accordance with the objects to which they are applied and with the mode of using them. For example, sheets are produced, having a constant thickness and having a constant length in the longitudinal direction or in the cross direction. However, even such standardized sheets may have a sag depending on the condition in production or during storage. This is because the sheet that shall naturally have a constant length from one end to the other may have a fluctuating length when tension is applied thereto at the two ends. For example, when the length in the longitudinal direction fluctuates, then the sheet may have a sag in the cross direction. When the sheet having such a sag is wound up, the wound roll may be deformed or wrinkled owing to the difference in the winding stiffness. When such a sheet is utilized or applied to objects, it may bring about various defects. For example, when used in printing, register marks could not be arranged in good order, and in multicolor printing, printing deviation may occur.
Such a sagging sheet brings about various problems, and therefore it is said necessary to evaluate the sag of a sheet during sheet production or before shipment. In particular, the industrial requirement is to evaluate the sag of a sheet while the sheet is conveyed in continuous production or continuous treatment thereof.
As a method for evaluating the sag of a sheet being conveyed, for example, proposed is a method of jetting out compression air toward a sheet through plural holes arranged at the top of the air table disposed above rollers for conveying a sheet thereon, thereby evaluating the degree of sagging of the sheet (for example, see Patent Reference 1). According to this method, air leakage occurs in accordance with the sheet tension between the air table and the sheet, and there occurs a pressure change in the air jetting port in accordance with the leakage. The pressure is measured with an air pressure sensor, and a pressure value is thus obtained in accordance with the sheet tension in the cross direction. The found data are plotted as a graph to be a tension profile in the cross direction of the sheet, from which the sag of the sheet in the cross direction can be evaluated.
As another method of evaluating the sag of a sheet being conveyed, there is known a method of using a double-structure roller comprising an inner cylinder and a large number of divided outer cylinders for evaluating the degree of sagging of a sheet (for example, see Patent Reference 2). According to this method, the outer cylinders are kept floating by the air fed through the air holes of the inner cylinder. When a sheet is made run while held around the roller, then the outer cylinders are pushed down by the force corresponding to the sheet tension. With that, the space between the outer cylinders and the inner cylinder changes, therefore producing a pressure difference between the upper part kept in contact with the sheet and the lower part (Pb) not kept in contact with the sheet. This is measured with an air pressure sensor, and from the difference, the value corresponding to the sheet tension is obtained. The found data are plotted as a graph to be a tension profile in the cross direction of the sheet, from which the sag of the sheet in the cross direction can be evaluated.
Patent Reference 1: JP-A 2001-33229
Patent Reference 2: JP-UM-B 3-11691